Most reported syntheses of Polylactic Acid (PLA) use enantiomerically pure Lactic Acid (LA) as raw material. As the raw material for the synthesis of LA is renewable, the commercial production of that biodegradable polymer is slowly gaining ground (Drumright et al., “Polylactic Acid Technology”, 2000, Adv. Mater., v. 12, pp. 1841-1846).
Its applications in the medical field may well command a high price, but if it is to compete successfully with fossil fuel derived polymers such as Polystyrene (whose mechanical properties are quite similar to those of PLA) or Polyethylene Terephtalate (for bottles), then its production cost has to decrease.
The synthesis reaction of PLA is a ring-opening polymerization of Lactide (LD) in the presence of a homogeneous catalyst (U.S. Pat. No. 5,319,107). The reaction conditions are now well known and there is apparently little to be gained by way of better product quality or smaller energy consumption in this step.
On the contrary, the classical production process from LA of the key intermediate, the cyclic diester LD is fraught with difficulties.
Several steps in the classical process are essentially single step evaporations (under low pressure) meant to remove solution water or condensation reaction water, as appears from the following description:                a.—Fermentation of a well-chosen carbohydrate by well-chosen bacteria, in the presence of Ca(OH)2 (or CaCO3) leads to the production of a suspension of bacteria in a solution of Calcium Lactate (CaLac2).        b.—the bacteria are separated by centrifugation or filtration and discarded (U.S. Pat. No. 5,766,439)        c.—the filtrate reacts with Sulfuric Acid, which causes the precipitation of Gypsum (Calcium Sulfate Dihydrate) and the liberation of Lactic Acid (LA) as a solution of some 10% by weight. (US Pat. Appl. No. 20050281913)        d.—that solution is concentrated by distillation to 85-88% LA by weight.        e.—the concentrated LA solution, in the presence of a homogeneous catalyst, undergoes a prepolymerization in a vacuum distillation column, where more water is separated.        f.—as the molecular weight of the prepolymer is only about 1000, its mechanical properties are not suitable for industrial use.        g.—the prepolymer is then depolymerized by back-biting in the presence of a well-chosen catalyst under vacuum and the LD leaving the reactor in the vapor phase is condensed as a liquid or directly fed to a distillation column to produce liquid crude LD (mixed with LA, some of its light oligomers, water, unwanted enantiomers of LD, etc. . . . ) (U.S. Pat. No. 5,357,035).        h.—the crude LD is further purified by liquid-liquid extraction with water followed by crystallization from aqueous solution (US Pat. Appl. No. 20060014975).        i.—centrifugation gives a cake of purified LD, but since the impurity level is still too large, a last operation is required:        j.—melt crystallization with sweating to remove the impurities by gravity flow.        
All these operations are well known, so that it is possible to produce for instance the enantiomer L-LD with a purity of up to 99.9%. But the yield of some of the operations of this long chain is rather modest, so that large recycle flows are required, so that the various equipment items tend to be large with large energy requirements.
A large Research and Development effort has been devoted to the improvement of the classical LD production process, i.e. the process where Lactic Acid is considered as the raw material (Rathin Datta, “Technological and economic potential of poly(lactic acid) and lactic acid derivatives”, 1995, FEMS Microbiology Reviews, v. 16, pp. 221-231). For instance, liquid-liquid extraction processes may offer an energetically less demanding way than water evaporation for the LA concentration, but complete elimination of traces of the solvent remains a problem. More advanced separation techniques, such as electrodialysis of ethyl lactate, are promising.
In the classical process, a large amount of waste product (Calcium Sulfate Dihydrate, or gypsum) is produced.
Other alpha-hydroxyacids, such as Glycolic Acid (GA), may similarly be dimerized to the corresponding cyclic diester and thence to the polyacid (e.g. glycolide leading to Polyglycolic Acid, PGA) by the same process and with the same disadvantages.
Notwithstanding these handicaps, PLA (and PGA) may well become a commodity product, so that the question of disposal or recycling of large quantities of waste material, such as empty bottles, must be addressed. A possibility is to depolymerize by heating at around 300° C., produce LD vapor and condense it. The process is very simple, but racemization at such a high temperature degrades the enantiomeric purity of the product. In order to by-pass this last difficulty, one may mix CaO or MgO powder to PLA and depolymerize by pyrolysis at lower temperature (Toru Motoyama, “Effects of MgO catalyst on depolymerization of poly-L-lactic acid to L,L-lactide”, 2007, Polymer Degradation and Stability, v. 92, pp. 1350-1358).
Especially relevant to the present invention is the observation that the depolymerization temperature (and the extent of racemization) of PLA with a molecular weight of 170 000 regularly decreases when the size of the MgO particles decreases (FIGS. 2 and 3 in Toru Motoyama, 2007). Also noteworthy is the fact that in said reference the MgO weight represents 5% (as Mg) of the weight of PLA.
Another point of interest in the former art is that Magnesium Hydroxide (Mg(OH)2) will dehydrate only at a much higher temperature (around 400° C.).
Yet another point of interest in the former art is that the linear dimer of LA, namely Lactoyllactic Acid (LacOLacA) is a stronger acid than LA (Bezzi, S., “I produtti di anidrificazione dell'acido lattico come tipo delle trasformazioni degli esteri ciclici in poliesteri lineari”, 1937, Mem. reale acad. Italia, Classe sci. fis., mat. e nat., v. 8, pp. 127-213).
It has long been known that the production of Magnesium Lactate (MgLac2) by fermentation is as easy as that of CaLac2 (U.S. Pat. No. 3,429,777). Moreover, since the solubility of MgLac2 is somewhat larger than that of CaLac2, the former may have an advantage if the Lactate is to be separated by crystallization.
If it were possible to go directly from the LA raw material, i.e. Calcium or Magnesium Lactate, directly to the LD, there would be no question of remaining traces of extraction solvent, nor of traces of depolymerization catalyst, and the production cost of PLA would probably decrease.
In a former patent application (Provisional Pat. Appl. 60/874,475), I have disclosed a process that would achieve these aims by bringing to the LD production reactor anhydrous reactants, such as Anhydrous Calcium Lactate and Sulfuric Anhydride, such that the organic product would have been dehydrated in a former operation under mild conditions.
The waste product obtained in said process may be used industrially, but a process without waste products would be even more attractive. Such a waste-less process is the object of the present invention.